Fructose, fruit and the right time to eat

Fructose: the specific caveat

Fructose is absorbed by the gut through the GLUT5 transporter (unlike glucose, which uses GLUT2). In the liver it is converted into glucose, lactate or triglycerides, depending on the metabolic context. The problem is not fructose itself. It is the amount and the context.

In excess (above 25 to 50 grams per day outside the sports context), fructose can be converted into triglycerides via de novo lipogenesis. This is a real risk for sedentary people on a hypercaloric diet rich in drinks sweetened with high fructose corn syrup.

For athletes training 8 to 15 hours per week, the story is different. With glycogen stores constantly depleted, fructose is a welcome raw material. A fundamental discovery by Jeukendrup and colleagues showed that combining glucose and fructose (2:1 ratio) allows oxidizing up to 1.7 g of carbohydrate per minute during exercise, versus 1.0 g/min with glucose alone.

The body is a machine: surplus becomes storage

The principle is simple but frequently misapplied: the body has limited capacity to store glycogen (about 400 to 500 g in the muscles and 90 to 120 g in the liver of trained adults). Any carbohydrate eaten beyond that capacity, when the stores are already full, will be converted into triglycerides and stored as fat.

Timing: the right time changes the result

Pre-workout (1 to 3 hours before): moderate to high glycemic index carbohydrates to maximize liver and muscle glycogen. Avoid excess fat and fiber (they delay gastric emptying and can cause discomfort).

During the workout (over 60 to 75 minutes): 30 to 60 g of carbohydrate per hour, or up to 90 g/h with a 2:1 glucose+fructose combination for long races. Water with electrolytes (sodium, potassium) to replace sweat loss.

Post-workout (within 30 to 60 minutes): glycogen synthesis window. Consuming 1.0 to 1.2 g of carbs per kg of body weight together with 0.25 to 0.3 g of protein per kg activates glycogen resynthesis and protein synthesis simultaneously.

The classic limit of about 1 g/min of oxidation came from saturation of the SGLT1 transporter, which carries glucose (and the maltodextrin that becomes glucose) across the intestinal wall. Fructose uses a different door, GLUT5, and afterwards both pass through GLUT2 into the bloodstream. By opening the two routes at the same time, the study by Jentjens and colleagues (2004) measured 1.26 g/min of oxidation with the glucose and fructose mix in a 2:1 ratio, versus 0.80 g/min with glucose only, at the same intake of 1.8 g/min. At even higher intakes (Jentjens and Jeukendrup, 2005), the peak reached 1.75 g/min.

This mechanism explains the so-called carbohydrate revolution in cycling. Tadej Pogacar told Peter Attia that five years earlier 120 g of carbohydrate per hour seemed impossible, and today his team uses 40 to 50 g gels of a glucose and fructose cocktail that sit well in the stomach. The Visma Lease a Bike team of Jonas Vingegaard works in the same range with a line at a glucose to fructose ratio of 1 to 0.8, although the team itself stresses that not every athlete tolerates or oxidizes 120 g/h, a number reserved for specific moments of a race.

What stays

Fructose is neither villain nor heroine, it is a tool: alone, the gut limits carbohydrate absorption to about 1 g per minute; combined with glucose in a 2:1 ratio, the ceiling rises to up to 1.7 g/min. This mechanism is what allowed cycling to move to a new level: Tadej Pogacar said that five years ago 120 g of carbohydrate per hour seemed impossible; today it is team routine. The difference between the banana that becomes fat and the banana that becomes energy is not in the fruit, it is in what the body is doing at that moment: the 400 to 500 g stored in muscle and liver are the ceiling, and it fills up fast in those who train little. Eat according to demand, not out of fear.